Small T antigen of SV40 is an early, virus-encoded protein which is believed to play an important part in the process of virus-encoded neoplastic transformation. The protein is encoded by a contiguous sequence from the proximal approximately 25 percent of the viral early region, and is, in part, a structural homolog of large T antigen. We propose to attempt to purify small T antigen to homogeneity and begin to characterize it as a protein. We also propose to attempt to probe certain aspects of its putative function in the transforming process using microinjection as a tool. In this regard, we will also try to evaluate the theoretical possibility that small T antigen might be a hormone-like molecule which can promote disordered cell growth by, in part, acting from outside the cell.